1) Field of the Invention
This invention relates generally to a versatile production system and method for operating the system. The system manufactures various kinds of products including semiconductor chips. The present system relates to methods for scheduling jobs on a production line to minimize the work in progress (WP). The system includes a measurement of how well jobs are distributed along the manufacturing line.
2) Description of the Prior Art
In general, in a production line for manufacturing products, a production control center controls the flow of jobs throughout the production line. In many production lines, the same types (sets) of tools can be used to process product at different steps. For example, in manufacturing semiconductors, product wafers undergo a general cycle of depositing a layer, covering the layer with a photoresist, etching the layer, removing the photo resist, and depositing another layer (or doping some exposed portion). Many of the equipment used to perform these operations can be used in several different product stages. For example, a photoresist patterning tool (e.g., optical exposers) could be used at the following product stages: field oxide photo, source/drain photo, gate electrode photo, contact photo, etc. Other equipment is used repeatedly, such as ion implanter, CVD tools, etch stations, etc.
A challenge of production line managers is to schedule or utilize the resources (such as equipment or personnel) in an efficient manner to maximize the product throughput. However, presently there isn't an effective method to determine the correct amount of jobs (WIP--Work in progress) and distribution of the jobs over the entire production line. The WIP will accumulate in some stages where it isn't really needed or useful. Many production supervisors make the decision to allocate local machines (and other manufacturing resources) based on their experience. These decision may further overload the sectors in some sections of the production line and under load the sectors in other sections of the line.
Practitioners have taken many different approaches to scheduling and monitoring manufacturing lines. For example, U.S. Pat. No. 5,446,671 (Weaver et al.) shows a look ahead method for maintaining optimum queued quantities of in-progress parts at a manufacturing bottleneck. U.S. Pat. No. 5,396,432 (Saka et al.) shows a production system to minimize line balance loss. U.S. Pat. No. 5,304,066 (Sontag) shows a machine shop management system for managing the work progress of production jobs.
There is still a need for a method of monitoring the WIP in the manufacturing line and determining the optimum amount of WIP at each stage.